Yahya related to me from Malik that Abu Jafar al-Qari said, "Iwas praying, and Abdullah ibn Umar was behind me and I was not awareof it. Then I turned round and he prodded me (in disapproval)." (PT)
USC-MSA web (English) reference: Book 9, Hadith 66 (PT)
Arabic reference (PT): Book 9, Hadith 397 (PT)